Metalloid precursor powders are useful in the making of ceramics. One such powder is a silicon powder which can be subjected to gas nitriding to form silicon nitride under proper heating conditions. Precursor powders have been made heretofore conventionally by casting an ingot of the precursor such as molten silicon, which ingot is subsequently crushed and ground to a powder condition. The molten liquid of which the ingot is cast contains residual impurities. These impurities, when localized as large, second phase particles, create flaws in the final ceramic which weaken the physical characteristics thereof. Coarse impurity localization is unavoidable in ingot-cast silicon; the second phases, such as iron silicide and aluminum in silicon, form at the grain boundries of the cast metalloid in localizations (second phase particles) that are sufficiently large in size that they can be seen with the unaided eye (50-200 micrometers and larger). Subsequent crushing and milling easily breaks down the matrix metalloid, but the intermetallic compounds, by virtue of being slightly malleable, can and do remain large. Air classification can, in principle, eliminate many of the largest localizations of impurities, but impurity localizations comparable to and larger than metalloid weight mean particle size (2-8 microns) can remain. Differences in ingot-casting technique (such as rotating the mold slowly versus stationary) and milling techniques (such as dry ball milling versus jet milling) produce only minor effects in the sense that impurity localizations that are established at the time of solidification tend to be difficult to remove.
This invention has found rapid cooling an important process adjunct to economically reducing in size or eliminating impurity localizations. Extremely fast cooling of liquid metals which could be useful as precursor elements has been studied in the last 20 years for a variety of objectives. A summary of glassy metal powder atomization is recited in U.S. Pat. No. 4,221,587. Similarly, fast casting of iron/silicon/boron alloys has been carried out (U.S. Pat. No. 4,386,896) to provide a transformer core material having improved property in the form of lower magnetic core loss. Each of these techniques do use fast cooling (not necessarily rapid cooling), but have incorporated metalloids only in minor amounts and only to produce glassy metals which have no crystalline peaks when examined by X-ray diffraction. Therefore, such patent teachings do not encounter the problem of concern here, namely, making a crystalline precursor powder for ceramics without localizations of intergranular second phases or impurities which can create flaws in the crystalline ceramic.
One attempt to use rapid solidification on metalloids used in a major amount is demonstrated in U.S. Pat. No. 4,347,199 (and related U.S. Pat. No. 4,419,060), wherein a high thermal transfer gas or fluid is used to cool droplets atomized by a rotating disc. Such technique has been developed solely with the aim of providing coarse and spherical powder particles which could subsequently be used in the making of silicone polymers. Such techniques did not encounter or appreciate the problem of impurity localization and therefore did not require the use of cooling which was sufficiently fast to avoid such local concentrations.
It is important to point out that slow rates of cooling promote impurity concentrations, and extremely slow rates of cooling are the basis of purification and zone refining techniques of silicon and a source of dendritic segregation and compositional coring in ingot-cast superalloys. Each of these results must be avoided if the precursor powder is to provide delocalization, that is, more uniform distribution of impurities or second phase additives. Use of segregation to produce purification of silicon has been described by Boulos in U.S. Pat. No. 4,379,777. The silicon is heated in a plasma and quenched. Upon solidification of the molten particles, a portion of the impurities migrates to the surface of the granules obtained. By iterative combination with leaching of surface segregated impurities, silicon of high purity is obtained. Segregation is also used in a related manner by the addition of aluminum metal in U.S. Pat. Nos. 4,193,974; 4,193,975; and 4,195,067, all assigned to the Union Carbide Corporation. Directional solidification of the molten material is achieved at a rate of 60.degree. C. per hour to achieve separate regions of solidified melt having high impurity concentration and another region having low impurity concentration.